The present invention relates generally to ophthalmic diagnostics, and more particularly to characterization of the retinal nerve fiber layer.
In the human retina, the optic disc is a specialized ocular structure in which the neural axons of the ganglion cells are gathered as bundles and leave the ocular globe. The retinal nerve fiber layer (RNFL) comprises ganglion cell axon bundles. The thickness of the RNFL increases as it gets closer in proximity to the optic disc.
Analysis of the morphological structure of the RNFL at and around the optic disc provides important clinical information for diagnosing diseases affecting the optic disc. In a healthy optic disc, a reference set of structures in the RNFL is present. One parameter characterizing the structures is the local thickness of the RNFL. In a diseased optic disc, such as that present in glaucoma patients, defects may correspond to changes in the RNFL structures. Some structures may be less pronounced, or, in some instances, absent altogether. Several ocular imaging modalities (for example, optical coherence tomography (OCT), scanning laser ophthalmoscopy, and scanning laser polarimetry) have been used to measure the RNFL thickness in vivo. Clinical studies have reported significant correlation between local variations in RNFL thickness with some ocular defects, for example, visual field loss.
As discussed above, various instruments may be used to measure the RNFL thickness. The RNFL thickness is dependent on the loci (points on the retina) at which the measurements are made. The RNFL thickness varies as a function of position at and around the optic disc. This functional dependence has been studied both by histology evaluation and by OCT. One method for diagnosing eye diseases is to compare the local RNFL thickness (at a set of measurement loci) of a patient's eye with a reference range of local RNFL thicknesses (at a corresponding set of measurement loci) measured from a population of healthy eyes.
Since the local RNFL thickness is a function of the measurement loci relative to the center of the optic disc, or other reference point in the retina, comparisons of the patient's data with the reference range must be determined at the same corresponding measurement loci. Errors will arise if the measurement loci in the patient's eye do not map properly to the corresponding measurement loci used for the reference range. For example, in the Zeiss Stratus OCT 3 instrument, a commonly used instrument in the field of ophthalmology, the RNFL thickness is measured at loci on a circle around the optic disc. The radius of the circle, relative to the center of the optic disc, is fixed at 1.73 mm. Since the precise center of the optic disc may be difficult to establish, and since there is typically eye movement during examination, in practice, the actual distance between the measurement loci and the center of the optic disc may vary from the target value of 1.73 mm. As a result, the reference range representative of the local RNFL thickness at a set of measurement loci in a population of healthy retinas may be broad. The resolution in detecting local variations relative to the reference range is therefore reduced.
What is needed is method and apparatus which has high sensitivity in detecting abnormalities in the RNFL and which has reduced sensitivity (relative to previous diagnostic techniques) to errors in the position of measurement loci. Method and apparatus which may utilize the existing reference range representative of the local RNFL thickness at a set of measurement loci in a population of healthy retinas are further advantageous.